Structure investigation of maltacine B1a, B1b, B2a and B2b: cyclic peptide lactones of the maltacine complex from Bacillus subtilis

A new complex of cyclic peptide lactone antibiotics from Bacillus subtilis, which we named maltacines, has recently been described. The structure elucidation of four of them is reported in this paper. The amino acid sequences and structures of the peptides were found by MSn of the ring-opened linear peptides that gave uninterrupted sequences of Bn and Y'n ions. The identities of three unknown residues in the sequences were solved by a combination of derivatization with phenyl isothiocyanate (PITC), high-resolution mass spectrometry and H/D exchange. The nature and position of the cyclic structure were revealed by a chemoselective ring opening with Na18OH and was found to be a lactone formed between a hydroxyl of residue number 4 and the C-terminal amino acid number 12. For verification of the structure of the B2+ ion, peptides with different combinations of P/Q and P/K at the N-terminus were synthesized. The structures of the four peptides were found to be as follows: B1a/B2a, cyclo-4,12(P-Q-Y-HNLeu-A-E-T-Y-Orn-103-Y-I-OH); and B1b/B2b, cyclo-4,12(P-Q-Y-HNLeu-A-E-T-Y-K-103-Y-I-OH).